The UN says Gaza's border crossings must be opened to allow humanitarian aid to be delivered to the strip's devastated population.
Aid agencies are rushing to get much-needed supplies into Gaza, following ceasefires announced by Israel and Hamas.
Some supplies are getting through, but the UN says much more is needed
John Ging, director of operations in Gaza for the UN relief agency Unrwa, told the BBC that delivering water was the first priority, and he called on Israel to open its border crossings with the enclave.
"Half a million people haven't had water since this conflict began," he said.
"The power lines, same thing, huge numbers cut off in wide areas. Housing - tens of thousands are homeless at the moment. We have a big recovery operation ahead of us. None of it will be possible until we can get the crossing points open."
Many Gazans have returned to find their homes and schools destroyed and are turning to the UN for help.
"Some areas are reduced to rubble so we have to find a temporary solution for them and also get the reconstruction operation under way," said Mr Ging.
On Monday, Israel agreed to let 143 trucks loaded with humanitarian aid into Gaza through crossings on the Egypt-Gaza border, plus 60,000 litres of fuel, a military official said.
Egyptian President Hosni Mubarak has announced plans to host an international aid conference to help rebuild Gaza.
During the fighting, distribution was hampered by security problems. The UN says basic shortages in the Gaza Strip remain, and prices of food to have risen considerably.
UN food agency the World Food Programme (WFP) says the price of chicken increased 23% percent; the price of wheat flour by 45%; the price of peppers by 100% and the price of tomatoes by 500%.
On 17 January, the WFP distributed 98 tonnes of food to 800 families in Gaza. Unrwa distributed food parcels to 874 families on 17 January.
The UN says hospitals in Gaza have been stretched to the limit, and medical personnel are under severe strain following more than three weeks of crisis.
The World Health Organisation (Who) says shell fire which hit the al-Quds Palestinian Red Crescent Society Hospital on 15 January has put it out of action. On the same day, the al-Wafa Rehabilitation Hospital in east Gaza City was damaged but continues to provide health care.
Patients have been evacuated from other hospitals to the Shifa Hospital, adding to its already overloaded capacity.
On 16 January, the International Committee of the Red Cross (ICRC) co-ordinated the delivery of more than 25,000 litres of fuel to 10 hospitals and clinics in Gaza City.
Fuel is needed for hospital generators as well as for ambulances. Recently, Shifa Hospital has been powered exclusively by generators to avoid power cuts, especially for patients in the intensive care unit.
During the Israeli attacks, about two-thirds of Gaza's 1.5m people were without power, the UN says.
Gaza's only power plant shut down on 30 December because it ran out of fuel.
On 18 January the plant received 90,000 litres of fuel from the filling depot at Nahal Oz in southern Israel, says the UN.
Since then most Gazans have had intermittent electricity, although some households are still without power due to damage to the grid.
However, the plant is still short of fuel. It needs 450,000 litres of industrial fuel per day to produce its full capacity.
Although most main power lines have been repaired, two lines are still damaged in northern Gaza.
WATER AND SANITATION
Some 400,000 Gazans still do not have running water.
Since 18 January an extra 100,000 people received running water when the electricity supply was reconnected.
Some wells have been refilled and several NGOs, including Save the Children, have distributed drinking water in the Gaza Strip. The UN says supplies could be improved if workers are given safe passage to repair three damaged water mains.
One water main east of Khan Younis has been damaged, cutting supplies to 25,000 people.
Officials have confirmed that all two million litres of wastewater at Gaza City's treatment plant, bombed on 10 January, leaked into surrounding agricultural land. Pumps at the plant are out of action due to lack of fuel.
A pump that sends sewage from Beit Hanoun to the Beit Lahia wastewater treatment plant is still damaged. According to officials, 30 cubic metres of sewage are flowing into the streets of Beit Hanoun every hour.
Unrwa says it is operating 50 emergency shelters for 50,896 displaced people in Gaza.
The shelters, many of them schools, are overcrowded with only basic levels of support, including food and water.
The shelters, especially those in the north, are in urgent need of non-food items and there is a shortage of more than 23,000 blankets and mattresses.
The UN says construction materials needed to repair and rebuild homes also need to be brought into Gaza.
The UN says Kerem Shalom, Karni and Rafah crossings were open on 18 January.
On 17 January, 51 truckloads including 10 for aid agencies entered Gaza via Kerem Shalom crossing, along with 115,000 litres of industrial fuel for the power plant.
Seven truckloads of medical supplies and a team of 27 medics were allowed into Gaza through Rafah. More than 20 casualties were evacuated from Gaza through Rafah.
On 16 January, 73 truckloads including 12 for aid agencies entered Gaza through Kerem Shalom, along with nearly 150,000 litres of industrial fuel for the power plant.
Twenty-one doctors, two ambulances and 10 truckloads of medical aid were allowed into Gaza through Rafah. Three casualties were allowed out of Gaza through Rafah.
The UN says the number of trucks allowed into the Gaza Strip needs to be increased. Additional crossings must be opened urgently for the provision of bulk grain, it says.